For many electronic amplifier applications, it is necessary that the amplifier assembly have wide bandwidth as well as high output power and low signal distortion. For example, in the field of radio-frequency transmission where low-level radio-frequency signals are amplified by wide bandwidth linear amplifiers of the distributed type. The amplified signals are then applied to antenna assemblies for aerial radiation. In these applications, the output power available from the amplifier must be sufficient to assure that the desired radiated field is achieved. Low distortion, moreover, is required to assure that the spectral components of the input signal are unmodified in the amplification process. But, typical amplifier configurations capable of providing the required wide bandwidth and power, such as the distributed amplifier, also introduce objectionable distortion.
A distributed amplifier is an amplifier system comprising a number of individual amplifying devices, such as vacuum tubes or solid-state devices. The input 3 terminal of each amplifying device is connected to an input line, and, correspondingly, the output terminal of each amplifying device is connected to an output line. The input and output lines of the distributed amplifying system are so designed as to be lumped-element transmission lines utilizing the parasitic reactive elements, capacitance and inductance, of the individual amplifying devices as part of the lumped parameters of these lines. Lumped-element transmission lines are well known in the art and are described in, for instance, U.S. Pat. No. 2,018,320, entitled "Radio Frequency Transmission Line," to Roberts. Lumped-element transmission lines applied to amplifiers are also described in the prior art in, for instance, U.S. Pat. No. 2,930,986, entitled "Distributed Amplifier," to Kobbe et al. In the distribution amplifier, the input signal to each successive amplifying device is delayed by the delay of the input line, and the delay of the output line is designed to be identical to that of the input line. This configuration results in each amplifying device delivering its output signal in phase with that of the other amplifying devices thereby providing maximum output signal delivered to the load. In effect, the collection of individual amplifying devices operate in parallel. However, if the amplifying devices were simply connected in parallel, the parasitic elements, most particularly the input and output capacitances respectively, would combine in parallel severely reducing the system bandwidth below that available from a single amplifying device. By including the amplifying devices in a distributed amplifier configuration, the effects of the parasitic elements of the amplifying devices do not add, and the bandwidth of a single amplifying device is preserved in the complete distributed amplifier system.
In the basic distributed amplifier of the prior art, the operating phase of each amplifying device is the same with respect to the output, i. e., a positive-going signal at the output terminal of the amplifier system is the result of a positive-going signal at each amplifying device output. Typically, the amplifying device produces an output signal that is asymmetric about the quiescent operating point, for example, the positive output signal component is somewhat lower in magnitude than the negative output signal component. This asymmetry is common in Class A operation of amplifying devices such as vacuum tubes, and is very distinct in operating classes of Class AB, Class B and Class C where the amplifying devices do not amplify the input signal over the complete input cycle. Such asymmetry results in distortion of the output signal in the basic distributed amplifier of the prior art.
Distortion in distributed amplifier systems results from the inherent parallel operation of the individual amplifying devices. Operation of a pair of amplifiers in a push-pull configuration is well known in the art as a means of reducing signal distortion. A push-pull distributed amplifier system is described in U.S. Pat. No. 4,337,439, entitled "Wide Band Amplifiers," to Sosin. The distributed amplifier system as taught by Sosin is composed of two individual distributed amplifiers connected in a standard push-pull configuration, i. e., two substantially similar amplifiers are interconnected at their input using a tapped transformer, and interconnected at their output using a second tapped transformer. Each amplifier must drive the parallel combination of the load and the output impedance of the companion amplifier. This typical push-pull configuration results in significant loading of each amplifier by the companion amplifier which reduces the power available for delivery to the load. Additionally, the required tapped transformers contained in the prior art by Sosin cannot be effectively fabricated in a manner provided wide-bandwidth performance and therefore introduce significant losses and frequency limitations. The configuration of the prior art by Sosin thus additionally results in severe bandwidth limitations over the basic distributed amplifier configuration. Since wide-bandwidth performance and maximum power delivered to the load are principle features of a distributed amplifier, a configuration inherently reducing bandwidth and delivered power is undesirable.
The present invention is significantly different from push-pull distributed amplifiers of the prior art as that taught by Sosin and referenced herein above. The present invention eliminates the companion amplifier loading and allows the required inverting means necessary for push-pull operation to be fabricated and installed in a manner consistent with wide-bandwidth performance.
Another variation of a push-pull amplifier in the prior art is described in U.S. Pat. No. 3,571,742, entitled "Push-Pull Distributed Amplifier," to Wengenroth. The amplifier as taught by Wengenroth is composed of several substantially standard push-pull amplifiers interconnected to input and output lumped-element transmission lines of a distributed amplifier configuration. The two individual amplifying devices in the individual amplifier stages in the configuration by Wengenroth are connected at their inputs by means of a center-tapped inductor. This center-tapped inductor is effectively an auto-transformer, i. e., a signal is applied to one-half of the total winding and signal is extracted from the total winding. The auto-transformed action thus provides equal magnitude and opposed phase input signals to the two individual amplifying devices of each individual amplifier as is common in the art of a standard push-pull amplifier. Similarly, the outputs of the two amplifying devices of each individual amplifier in the configuration by Wengenroth are combined in a center-tapped inductor, also effectively an auto-transformer. Thus, each individual amplifier stage in the configuration by Wengenroth is a substantially standard push-pull amplifier: two substantially similar amplifying devices interconnected at their inputs using a tapped transformer, and interconnected at their outputs using a second tapped transformer. A disadvantage of this configuration by Wengenroth is that the parasitic elements, particularly input and output capacitance, of the two individual amplifying devices comprising each amplifying stage are effectively connected in parallel causing a reduction in bandwidth over that available from a single amplifying device. Another disadvantage of the configuration by Wengenroth is that the auto-transformer configuration of the input and output tapped inductors results in an electrical configuration of poor electrical symmetry due to the additional parasitic elements introduced by the auto-transformer. Thus, the source impedance driving the two input terminals of the two individual amplifying devices are not equal, and similarly the output impedance presented to the two output terminals of those amplifying devices are not equal. Such asymmetry results in poor performance over frequency and reduces the wide-bandwidth capability of the distributed amplifier system. A further disadvantage of the amplifying system by Wengenroth is that the required tapped inductors cannot be fabricated in a manner providing wide-bandwidth performance. For example, the tapped inductors cannot be fabricated with a well defined broadband characteristic impedance, such as 50 ohms or 100 ohms, since such impedance would present loading in both the input and output lines of the overall distributed amplifier system. That loading would make it impossible to properly effect lumpedelement input and output transmission lines of low-loss, wide-bandwidth performance as is necessary for operation of the distributed amplifier configuration. Still another disadvantage of this configuration is that input and output tapped inductors are needed for each amplifying stage. The physical size of these tapped inductors and the required position in the circuit introduce additional parasitic elements (capacitance and inductance) that severely limits the bandwidth and the total number of stages that may effectively included in the distributed amplifier system. The principle advantage of the distributed amplifier configuration is the effective use of many individual amplifying stages to provide higher output power than each single stage while preserving the bandwidth available from a single amplifying stage. Therefore, the configuration taught by Wengenroth does not allow the principle value of the distributed amplifier to be realized.
The present invention provides an improved pushpull distributed amplifier system in which individual amplifying devices are arranged in two or more groups. The amplifying devices within each group operate in phase and groups of amplifying devices operate 180 degrees out-of-phase with means included for signal inversions required for accurate in-phase adding of the output signals of each group. The means utilized in the present invention for providing the required signal inversions may be fabricated with very well-defined, low-loss, broad-band characteristics accurately matching both the characteristic impedance and bandwidth performance of the input and output transmission lines of the distributed amplifier configuration. One such construction of the signal inverting means is that of a transformer wound with high-quality transmission line of the impedance required to match the signal line to which it is to be applied.
The present invention is substantially different from distributed amplifiers of the prior art as taught by Wengenroth and references herein above. By Wengenroth, a plurality of substantially standard push-pull amplifiers are utilized as the individual amplifying elements of a distributed amplifier configuration. Thus, by Wengenroth, push-pull operation is provided in each of the individual amplifying devices of a distributed amplifier configuration. In contrast, in the present invention configured with more than one distributed amplifier, a plurality of individual distributed amplifiers are combined by the substance of the invention to effect push-pull operation.
The present invention may also be applied to a single distributed amplifier. A configuration of a single distributed amplifier with only two amplifying devices and further configured according to the present invention may be compared to the individual amplifying stage by Wengenroth. The individual stage by Wengenroth is electrically a generally standard push-pull configuration suffering the disadvantages as previously reviewed herein above. In contrast, the present invention configured with a single distributed amplifier of only two individual amplifying devices fully retains the electrical properties of the basic distributed amplifier configuration. Therefore, the present invention effectively incorporates the parasitic elements of both the amplifying devices and the signal inverting devices in very well-defined and properly terminated transmission line structure which preserve the very broad bandwidth performance of the distributed amplifier and in addition provides push-pull operation. Thus, the present invention over Wengenroth, applied to a distributed amplifier with as few as two amplifying devices, retains all of the advantages of the distributed amplifier configuration while providing the added advantages of push-pull operation.
This improved configuration provides push-pull operation in a distributed amplifier configuration without compromise of output power and bandwidth inherently introduced by configurations of the prior art. Further, the number of amplifying devices that may be effectively employed and the frequency performance of the basic distributed amplifier is not compromised by use of this improved configuration. This allows the full benefit of the distributed amplifier configuration to be realized. The phase-opposed operation of the various amplifying devices with accurate in-phase addition of their output signals at the output terminal of a distributed amplifier configuration as embodied herein is a novel push-pull configuration which provides lower distortion, higher output power, and thus higher efficiency. Moreover, because of the unique push-pull configuration of the present invention, some of the power previously lost due to loading by the companion amplifier, as taught in the prior art, is made available to the load. The present invention thus provides as much as a factor of two improvement in the output power available from a distributed amplifier as taught in the prior art. By providing higher available output power, the present invention provides lower distortion for any specific output power. It is well known that distortion increases when an amplifier is driven to its maximum output capabilities. Since the amplifier of the present invention, for any specific delivered output power, operates at lower power level with respect to its maximum capabilities than an amplifier of the prior art composed of a similar number of amplifying devices, the present invention will provide lower distortion. Further, for any specific desired maximum output power, the electrical size of the amplifiers of the present invention may be reduced by a factor of as much as two over the electrical size of the amplifiers required in the prior art. The input quiescent operating power of the present invention is therefore reduced over the prior art for an amplifier configuration capable of a specific maximum output power. The reduced amplifier quiescent operating power of the present invention therefore provides increased efficiency. The present invention also provides reduced cost over the prior art for an amplifier of specific maximum output power capability because the cost and the electrical size of the amplifier are directly related.